Revelation 14:3 (ESV)

3 and they were singing a new song before the throne and before the four living creatures and before the elders. No one could learn that song except the 144,000 who had been redeemed from the earth.

The pronoun “they” could refer to the 144,000, yet that could not be correct since they are the ones who need to learn this song (Revelation 14:3). Perhaps “they” refers to the 24 elders who held harps in Revelation 5:8, yet that too seems unlikely since their song happens “before” (in front of) the throne (of God) and the four living creatures and the 24 elders. The best is to assume that heavenly choirs of angels and the saints who have already entered the presence of God join in singing in response to the leadership of the harps (as the saints of Israel present in the temple compound did in response to the Levites playing the harps).